Since RNA is transcribed by complementary pair matching, and DNA is double stranded, does that mean that only one side is used each time? Since the other side would result in a different sequence of bases? If so, how does the RNA polymerase know which side, and does that mean that only one strand is actually used, and the other strand is purely for replication purposes?

transcription initiation only occurs at the strand that is defined by specific sequence patterns, as jackbean already mentioned. At the key of this patterns are promotors, which lie 5' upstream of each gene. Promotors mediate interactions between DNA binding proteins and the RNA polymerase, whereby transcription is initiated. So of course will the transcription of both strands results in different polypeptides, but it is important to consider if both strands are actually transcribed inside the cell...